A laminated glass is a safety glass which, even when broken by impact from the outside, shatters into few flying glass fragments. For this reason, a laminated glass is widely employed as the windowpanes of vehicles such as cars, aircrafts, buildings, and the like. Examples of a laminated glass include a laminated glass produced by interposing between at least a pair of glass plates an interlayer film for a laminated glass, which contains a plasticizer and a polyvinyl acetal resin such as a polyvinyl butyral resin.
In recent years, attempts have been made to reduce the thickness of laminated glasses in order to reduce the weight of the laminated glasses. However, reduction in the thickness of laminated glasses deteriorates the sound insulation. When such a laminated glass is used for the windshield of a car or the like, the sound insulation is insufficient against sounds such as wind noise and wiper noise which are at a frequency in an approximate range of from 2000 to 5000 Hz.
In view of such a sound insulation problem, Patent Document 1 for example discloses an interlayer film for a laminated glass having a middle layer, containing a polyvinyl acetal resin and a large amount of a plasticizer, sandwiched between two protective layers containing a small amount of a plasticizer and a polyvinyl acetal resin that contains acetyl groups in a small amount compared to the polyvinyl acetal resin contained in the middle layer. Such an interlayer film for a laminated glass is considered to provide high sound insulation.
The laminated glass obtained by using the interlayer film for a laminated glass described in Patent Document 1 indeed provides very high sound insulation in the normal temperature range around 25° C. However, cars and the like are used in an environment with temperatures usually in the wide range of from 10 to 60° C. Hence, the laminated glass obtained by using the interlayer film for a laminated glass according to Patent Document 1 provides insufficient overall sound insulation at temperatures in such a wide range.